This application claims the priority of German application 196 52 709.0, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a transfer system and, more particularly, to a system for transporting workpieces along press stations and the like having at least one transfer rail whose longitudinal course coincides essentially with a direction into to which the workpieces are to be transported in a timed manner and which is provided with devices for receiving and holding the workpieces, having several electric drives whose outputs are connected with the transfer rail and which drive the transfer rail with a coinciding power direction so that the electric direct drives form a driving unit for a predetermined axial direction, having a control device which controls the electric direct drives pertaining to the driving unit essentially in a coinciding manner and which takes up an emergency operating mode in the case of danger caused by the failure of a direct drive or parts of the control device assigned to it.
Particularly multistation presses, such as vehicle body presses or the like, have transfer systems for transporting workpieces from station to station. For this purpose, such transfer systems usually have two mutually parallel transfer rails between which mutually spaced cross traverses are held which have holding devices, such as suction spiders or the like. The workpiece transport takes place by a combined lifting, advancing and lowering movement of the transfer rails, i.e two-axis transfer.
Furthermore, transfer systems are known in which, instead of the cross traverses, holding devices are provided which are held on the transfer rails. In addition to the lifting, lowering and advancing movement, the cross traverses also carry out a transverse movement, i.e. three-axis transfer, with a vertical movement for the lifting/lowering, a longitudinal movement for carrying out a transfer step, and a transverse movement for the opening and closing.
A three-axis transfer is known in which linear motors are provided as drives for the movements in the three moving axes. The linear motors are controlled separately, in which the linear motors of one transport rail are controlled synchronously with the linear motors of the other transport rail as well as synchronously with the press.
While the direct drive of transfer drives by using linear motors results in low-cost, constructionally simple transfer systems, which permit a flexible application, in the event of a failure of individual system components, there is the danger that the transfer rail and/or other elements of the transfer system, such as cross traverses, do not carry out the desired movement and, for example, do not leave the work station in time after the receiving and depositing of a workpiece. In the case of multistation presses, this may lead to serious damage to the transfer rail or other elements and, in the worst case, to the entire transfer system and/or the tools.
DE 44 22 719 A1 describes a safety device for a flexible transfer system for presses, in which the transfer rail of a three-axis transfer is disposed on a prestressed spring-loaded device. This spring-loaded device is locked in the inoperative condition in its tensioned position and, when it is triggered, generates a fast lateral movement of the transfer rail. For the locking of the spring-loaded device, a toggle lever system is triggered electromagnetically in the event of a defect. The now occurring fast transverse movement of the transfer rail leads this transfer rail out of the danger zone.
For a single transfer rail, several spring-loaded devices are as a rule required which, overall, result in significant expenditures and in a considerable additional mass which must be accelerated and braked during the transfer.
It is an object of the present invention to provide a fail-safe transfer system which has good dynamics.
This object has been achieved in accordance with the present invention by a transfer system in which in the emergency operating mode, the control device 36 essentially disconnects the direct drive of the drive unit from power which is affected by the failure, and in that, in the emergency operating mode, the control device continues to control the remaining direct drives and moves the transfer rail and its holding devices out of the work stations.
The transfer system according to the invention, which may be constructed as a two-axis or three-axis transfer, has at least one, preferably two mutually parallel transfer rails which are operated in at least one axial direction, e.g., in the "opening/closing" direction, that is, toward one another and away from one another, by means of electric direct drives. Thereby, the respective transfer rail is driven in the transverse direction by several electric direct drives which act in parallel and which are applied to the transfer rail by means of their respective output. The direct drives are spaced away from one another and are distributed along the length of the transfer rail. For example, three or more direct drives are provided for each axis.
The control device provided for the control synchronously controls the direct drives pertaining to one axis in a coinciding manner so that so that the transfer rails carry out the predetermined movement over their entire length in a uniform manner. The control device preferably receives sensor signals, for example, concerning the actual position of the transfer rail, so that a position control can be achieved.
In addition to its regular operating mode, in which it causes the transfer rail(s) to move at a predetermined timing along a preferably programmable or otherwise adjustable transfer curve, the control device has an emergency operating mode in which it disconnects at least one of the connected direct drives, which are to be operated synchronously, from power or essentially from power. Such an error may, for example, occur when a position sensor assigned to the corresponding linear drive emits an invalid signal. When this is recognized, the assigned direct drive is disconnected from power very rapidly. As a result, the assigned direct drive is prevented from operating against the other direct drives and block or hinder the movement of the transfer rail. In the event of the failure of a single drive, the sequence of movements can be continued until a secure condition is reached.
The communication system (bus, star, ring) between the central control and the intelligent drives is configured such that, in the event of the failure of an individual drive, it will continue to remain operative. However, the control device will continue to control the remaining direct drives in the emergency operating mode so that the movement of the transfer rail will at least be continued until the holding devices are guided out of the work stations. Because of the fast power disconnecting of the direct drive affected by the damage, this can take place by means of the remaining linear motors at a sufficient speed, these being desired for this purpose.
In particular, when the transfer rail is moved in one axis by several, preferably more than three direct drives, the performance reduction in the case of the failure and after the power disconnection of a direct drive will be low so that the transfer rail can be moved out of the danger zone. Optionally, the direct drives can also be operated for a short time at a higher power in the emergency operating mode. In particular, this can take place when the operation of the transfer system is not continued for several cycles but only until the transfer rails and/or the holding devices are in a safe position.
The direct drives assigned to one axis preferably have a uniform capacity. Thereby, in the event of a failure of any direct drive, the emergency operation will in each case result in tolerable conditions. The capacity of the direct drives is preferably dimensioned such that, in the event of a failure of one direct drive, they will still supply the full required total power or at least sufficient power.
The transfer rail of the transfer system according to the invention is preferably constructed to be particularly resistant to bending in order to ensure that, also in the event of a failure of one direct drive, no elastic bending will occur which could lead to a collision between the transfer rail and the tool. For this purpose, the resistance to bending, for example, in the lateral direction is increased. Consequently, the natural frequency of a supporting rail section which extends from an operating direct drive by way of a defective direct drive to the next operative direct drive is higher than the lowest excitation frequency which results from the lateral movement in the emergency operating mode.
In the emergency operating mode, the movement of the transfer rail may deviate from the movement curve which the transfer rail travels during the regular operation. In particular, the movement curve during the emergency operation may be defined such that a fastest possible leaving of the work stations is permitted. Under certain circumstances, this may be achieved by a pure lateral movement of the transfer rail. As a rule, however, also during the emergency operation the curve travelled in the regular operation is attempted to be followed at least within a predetermined tolerance.
Direct drives are preferably electric linear motors; however, hydraulic or other electrically controllable direct drives can also be used within the scope of the present invention.
For sensing failures on sensors, the direct drives or the control device, a monitoring device can be provided for recognizing defect conditions. The monitoring device can be part of the control device or can be constructed separately therefrom. In the event of a failure in the system, the monitoring device determines the direct drive which can no longer be controlled correctly and initiates the emergency operating mode by way of the remaining direct drives.
A further increase of the operating safety is achieved by the use of a redundant communication system between the central control and the intelligent drives. The fail-safety is further increased by the redundant construction of the central control.
The control device preferably contains separate control units which are each assigned to a direct drive. The control units which are subordinate to a central unit communicate with one another and with the central unit by way of a data transmitting system which may have a ring structure or a bus structure. For avoiding dependent failures of larger system parts, failed transmission sections may be switched inactive. In addition, the entire system or parts thereof can be constructed to be redundant in that stand-by units are provided which take over the operation in the event of a failure.
The advantages of the transfer system as well as advantageous embodiments are also correspondingly achieved for a multistation press which is equipped with such a transfer system.